1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus. More particularly, the present invention relates to an image forming apparatus capable of reducing the influence of the variation of rotation speed of an image carrier, which is ascribable to, e.g., the eccentricity of a driven transmission member mounted on the image carrier, on the misregister of different colors as well as the influence of the load variation of the image carrier acting on transfer medium drive means on the misregister of different colors.
2. Description of the Background Art
Today, to meet the increasing demand for color copies and color prints, an ink jet image forming apparatus is predominant in a low-speed range while an electrophotographic image forming apparatus is predominant in a medium- and a high-speed range. Particularly, a tandem color image forming apparatus including a plurality of photoconductive drums or image carriers arranged side by side in the direction of sheet feed is suitable for high-speed applications. On the other hand, an intermediate image transfer type image forming apparatus includes an intermediate image transfer body implemented as a belt and configured such that an image formed on the belt is transferred to a sheet.
Japanese Patent Laid-Open Publication No. 10-246995, for example, discloses a tandem color image forming apparatus including four photoconductive drums arranged in a direction in which a sheet carried on a conveying belt moves. An optical scanning unit scans each of the drums with a light beam in accordance with image data in the main scanning direction, thereby forming a latent image on the drum. In this manner, latent images are formed on all of the four drums. A developing unit storing toner of particular color, i.e., cyan, magenta, yellow or black develops preselected one of the latent images. The resulting toner images are sequentially transferred from the drums to the sheet one above the other by chargers, completing a full-color image on the sheet. The sheet with the full-color color image is driven out of the apparatus after the toner image has been fixed on the sheet.
As stated above, in the tandem color image forming apparatus, toner images of different colors are formed on the drums in parallel and complete a full-color image only if the sheet is passed through consecutive image transfer positions only one time. The tandem color image forming apparatus is therefore feasible for high-speed color image formation.
In the tandem color image forming apparatus, a specific drive system for driving the drums is configured such that the drive force of a pulse motor, which is not subjected to feedback control, is transmitted to a driven gear or driven transmission member mounted on the shaft of the individual drum. When such a drive system is used for accurate drum drive, it is likely that the speed variation of the individual drum brings about the misregister of pixel positions transferred to the sheet, belt or similar transfer medium, resulting in the misregister of colors in the full-color image.
The major causes of the drum speed variation are (1) the variation of rotation speed of each gear or similar drive transmission member included in a drive transmission system between the drive motor and the driven member of the drum shaft, and (2) the eccentricity of the drum shaft or that of the driven gear as well as the cumulative tooth pitch error of the driven gear. The speed variation of the drum ascribable to the cause (1) tends to occur at a period shorter than a period of time necessary for the drum to make one rotation. On the other hand, the speed variation of the drum ascribable to the cause (2) tends to occur at a period coincident with the above period of time.
Japanese Patent No. 2,929,671, for example, teaches a method of obviating the drum speed variation ascribable to the cause (1). Assume that oscillation included in the variation of the drum angular velocity has a frequency fi, and that a fundamental frequency dependent on a period of time necessary for the drum to rotate over an angle θ1 about its axis O from an exposure position to an image transfer position is fo. Then, the method taught in the above document makes fi/fo an integer. More specifically, assuming that the above oscillation has a period T, and that the above period of time is τ, then the method makes τ/T an integer. Therefore, even when the drum angular velocity varies due to the oscillation fi ascribable to, e.g., the eccentricity of the gear, images can be brought into register on the transfer medium.
However, when it comes to the drum speed variation ascribable to the cause (2), the method described above cannot satisfy a condition that makes the parameter fi/fo or τ/T an integer. It is therefore difficult to obviate the misregister of colors ascribable to the drum speed variation.
In the tandem color image forming apparatus, the variation of torque ascribable to the torque ripple or the cogging of the drive motor assigned to the drum is magnified by a speed reduction ratio implemented by the drive transmission system before it is transferred to the driven gear of the drum. To solve this problem, Japanese Patent Laid-Open Publication No. 10-63059 proposes a color image forming apparatus using a transmission mechanism that reduces the speed of a drive motor with gears, and mounting a large flywheel on the shaft of a drum that is the subject of control. With this configuration, the apparatus reduces oscillation generated in, e.g., the drive transmission system. However, although the flywheel may reduce high-frequency oscillation ascribable to, e.g., gears, it cannot easily reduce drum speed variation ascribable to the eccentricity of gears constituting the drive transmission system. As a result, the rigidity of the drive transmission system is lowered to make accurate control difficult to execute.
Japanese Patent Laid-Open Publication No. 63-11967, for example, discloses an image forming apparatus constructed to obviate the misregister of images transferred from the drums to the transfer medium. In this image forming apparatus, at least two of a plurality of drums share a single drive means. The drums are positioned such that a period of time necessary for a conveying belt to move between image transfer positions assigned to nearby drums corresponds to a distance equal to the integral multiple of the period of the drive irregularity of the shared drive means. This kind of apparatus is effective so long as, when the drive irregularities are applied to the drums in the same phase, all the drums are driven by, e.g., a single drive motor, i.e., the speed variation of a gear mounted on the output shaft of the drive motor is transferred to all of the drums. However, because consideration is not given to the influence of a difference in phase between the speed variations of driven gears mounted on the drums, it is difficult to obviate color misregister ascribable to the speed variation of the individual drum resulting from the eccentricity or the cumulative tooth pitch error of the driven gear, which is mounted on the drum shaft.
Further, the apparatus taught in the above Laid-Open Publication No. 63-11967 or Japanese Patent No. 2,929,671 can match the drums with respect to the image position against the speed variation of the individual drum, which is ascribable to the eccentricity or the cumulative tooth pitch error of the gear of the drive transmission system that is not directly connected to the drum shaft. However, when speed variation occurs at each of the drums, slips ascribable to the speed variations are superposed at the consecutive image transfer positions. This is likely to bring about the thickening of lines or similar defects.
Japanese Patent Laid-Open Publication No. 8-160690, for example, proposes an image forming apparatus using a direct drive system including an ultrasonic motor. The ultrasonic motor makes the transmission gear and driven gear unnecessary and thereby obviates the drum speed variation ascribable to the gear of the drive transmission system and the driven gear of the drum. The ultrasonic motor, however, has the following disadvantage well known in the art. In the ultrasonic motor, a rotor is held in contact with a stator. Therefore, the rotary body of the ultrasonic motor must be configured to be freely rotatable when the drum is replaced, thereby protecting the motor from damage. This is also true when a sheet jamming a printer or a copier should be removed. The ultrasonic motor therefore increases the cost of the apparatus.
While a core motor with coils wound round the slot yoke of a stator or a pulse motor is a common motor that can be directly connected to the drum, such a motor involves, e.g., cogging. If the core motor or the pulse motor is directly connected to the drum, cogging, for example, directly translates into the speed variation of the drum. To solve this problem ascribable to cogging, the drive system may use an outer rotor type coreless motor in order to reduce the high-frequency speed variation with an inertia effect available with this kind of motor. However, it is difficult with this drive scheme to obviate the influence of a transitional load variation occurring when, e.g., a sheet rushes into contact with the drum.
In the tandem color image forming apparatus, assume that the drum speed varies due to the eccentricity of the drum shaft or the eccentricity or the cumulative tooth pitch error of the driven gear or that the drum itself is eccentric. Then, a difference between the drum peripheral speed and the speed of the intermediate image transfer belt or a difference between the drum peripheral speed and the conveying belt and sheet speed varies at the image transfer position. The resulting friction acting between the drum and the intermediate image transfer belt or between the drum and the conveying belt and sheet at the image transfer position varies. Consequently, a load acting on the intermediate image transfer belt or the drive system assigned to the conveying belt varies, causing the speed of such a belt to vary.
Japanese Patent Publication No. 6-13373 teaches a sheet conveying device constructed to reduce the influence of the eccentricity of a drive roller, which drives a conveying belt, on color misregister. To achieve this purpose, the distance of a sheet path between nearby drums is selected to be the integral multiple of a distance by which a sheet is conveyed for one rotation of the drive roller. By applying this sheet conveying device to an image forming apparatus, it is possible to obviate color misregister ascribable to the speed variation of the conveying belt resulting from the eccentricity of the drive roller. However, the sheet conveying device does not give consideration to the speed variation of the drum ascribable to the eccentricity or the cumulative tooth pitch error of the driven gear coaxial with the drum shaft or the eccentricity of the drum itself. Further, the above document does not teach specifically any condition that reduces color misregister when a driven transmission member is interposed between the drive roller and a drive source for saving power to be consumed by a drive system assigned to the drive roller.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-179445, 10-333398, 11-59947, 2000-162846, and 2000-227738.